For refrigeration system designers, integrators and O&M personnel, oil return efficiency is a key factor affecting compressor life, system stability and energy consumption. Poor oil return is easy to lead to compressor wear, lubrication failure, but also increase operating costs, which is a lot of refrigeration units generally face problems.
In this paper, we will analyze the core principle, core advantages and application scenarios of the three common oil return methods in refrigeration units to help you choose a more appropriate oil return program for the system.
Ejector Oil Return Technology
Ejector oil return technology is the core component of the oil return system, and its working principle is based on the fluid dynamics of the priming effect: refrigerant flow through the nozzle at high speed to form a low-pressure area, resulting in suction adsorption of lubricating oil.
The lubricant is first mixed with the refrigerant through the pipeline or oil separator, and then the ejector will lead the lubricant in the mixed fluid out of the low-pressure area to the compressor suction port.
The system is usually equipped with a filter drier and an oil sight glass to prevent impurities from entering the compressor and causing damage, and to monitor the oil return status in real time.
Core Advantages
- Efficient oil return: With the refrigerant’s own kinetic energy to realize the oil return, without the need for additional external oil pumps or complex mechanical devices, even in complex refrigeration systems, can be efficiently brought back to the compressor, to ensure that the system continues to lubricate.
- Significant energy and cost advantages: compared with the traditional oil return pump system, no additional electric power drive, only rely on the system internal fluid power can run; through the solenoid valve and angle valve can adjust the primary flow, further optimize the efficiency of oil return, reduce energy consumption and operating costs.
- Extremely adaptable: applicable to all sizes of refrigeration systems, especially in long pipelines or complex layout scenarios, it can highlight the advantages of oil return and effectively solve the problem of oil return in long distance and complex paths.
Direct Oil Return Technology
Direct oil return technology core: through the optimization of piping design, so that the lubricating oil and refrigerant in the evaporator mixing, through the throttle plate or electronic expansion valve flow control, direct return to the compressor suction side, without the need to configure the oil and gas separator.
It should be noted that the oil return method requires strict control of the oil return volume, to avoid excessive lubricant into the compressor to cause liquid compression failure; the actual application can be optimized through the monitoring of exhaust temperature control accuracy.
Core Advantages
- System Design Optimization: The elimination of key auxiliary equipment such as oil separator and oil return pump significantly reduces the complexity of the overall design of the system, while streamlining the piping connection nodes to make the system structure more compact, and to enhance the convenience of installation and later transformation.
- Double reduction in cost and energy consumption: Significantly reduce the initial investment in equipment procurement, while reducing the subsequent maintenance of equipment, replacement of operation and maintenance costs; no additional power for the oil return equipment, eliminating the related energy consumption, and effectively improve the overall energy efficiency of the refrigeration system.
- Efficient and stable oil return: Relying on the precise piping design and flow control, it ensures that the lubricating oil flows back to the compressor quickly and smoothly, providing continuous and reliable lubrication protection for the core components of the compressor, and avoiding the risk of failure due to lubrication failure such as wear and tear of the components and dry wear from the root.
- Improved operational reliability: By streamlining the number of mechanical components, the potential nodes for failure are reduced, reducing the probability of unplanned system downtime; at the same time, the simple structure also makes it easy for operation and maintenance personnel to quickly troubleshoot problems, further improving the long-term stability of the system.
Gravity Oil Return
The core design logic of gravity oil return is to rely on gravity to realize the natural return of lubricant: through the scientific design of pipeline slope and height difference, to guide the lubricant from the high point of the system to the suction end of the compressor.
When adopting this return method, the evaporator should be installed above the compressor to form a sufficient height difference to provide return power. After mixing with the refrigerant, the lubricant is separated in the evaporator and returned to the compressor by gravity through optimized oil channels.
Unique Advantages
- Simple system structure: no need to configure additional oil pumps and other complex equipment, significantly reducing the overall complexity of the system, and later maintenance is more convenient, significantly reducing the workload of operation and maintenance.
- Remarkable energy-saving effect: completely independent of external power equipment, relying on gravity to naturally drive the oil return, significantly reducing the system energy consumption, long-term operation can save considerable energy costs.
- High operational reliability: because it does not rely on external equipment and complex mechanical structure, the failure rate is lower, and the operational stability is stronger; through the high-pressure/low-pressure float valve or electronic expansion valve to accurately control the evaporator level, to ensure stable and controllable oil return process.
Comparative Analysis of Three Oil Return Methods
Reliability
Reliability level, gravity oil return by virtue of gravity-driven design, no additional mechanical transmission components, the failure rate is relatively lowest; direct oil return eliminates the oil separator, oil return pump and other key auxiliary components, the risk of mechanical failure is in the middle;
ejector oil return needs to be configured with filters, sight glass and other auxiliary components, but the core component ejector structure is simple, wear-resistant, the overall operational stability still maintains a high level. The overall operational stability remains high.
Cost and Long-term Investment Value
In terms of cost and long-term investment value, gravity oil return does not require additional auxiliary equipment, so the initial investment and later operation and maintenance costs are the lowest; direct oil return eliminates some key equipment, so the cost is at a medium level, but it requires more efforts in pipeline optimization design and flow rate control;
Pilot oil return is equipped with a special pilot and ancillary auxiliary components, so the initial investment is slightly higher, but from the long-term operation point of view, it has significant energy saving. However, from the perspective of long-term operation, its significant energy-saving advantages can offset part of the initial investment, and the value of investment is gradually highlighting.
System Complexity
At the level of system complexity, gravity oil return is the most simple structure, with the lowest requirements for system configuration; direct oil return requires key optimization of piping design and flow control, with a medium level of complexity; and
inducer oil return requires precise matching of inducer parameters, design of low-pressure zone piping, and supporting auxiliary monitoring components, with the highest requirements for configuration, but the overall scheme is still easy to operate and debug.
Applicable System Size and Scenarios
Applicable system size and scenarios, gravity oil return is more suitable for small, short pipeline and simple layout of the refrigeration system, especially for the evaporator can be arranged in the compressor above the scenario;
direct oil return for small and medium-sized refrigeration systems, the flexibility of the pipeline layout of the requirements of higher, more suitable for the pursuit of a streamlined system, the cost of controllable project requirements;
ejector oil return for all scales of Refrigeration system, especially in the long pipeline, complex layout of medium and large refrigeration units, can give full play to its oil return advantage, with irreplaceable.
How to Choose Suitable Oil Return Method for Refrigeration Unit?
System Capacity
System capacity is the basis for choosing oil return method. For small refrigeration systems (such as small commercial freezers, household central air conditioners, etc.), because of its short pipeline, stable load and sensitive to cost control, you can give priority to gravity oil return or direct oil return, not only to meet the basic oil return requirements, but also to reduce the investment in equipment and operation and maintenance costs;
And medium and large refrigeration systems (such as industrial chiller, large cold chain storage refrigeration system, etc.), the oil return path is longer, the oil circulation is larger, and the oil return efficiency and stability can be fully utilized.
As for medium and large refrigeration systems (e.g. industrial chiller units, large cold chain storage refrigeration systems, etc.), the oil return path is longer, the oil circulation volume is larger, and the requirements for oil return efficiency and stability are higher.
Compressor Types
Different types of compressors have different structures and working principles, and have very different requirements for oil return speed and oil volume. For example, scroll compressors require high oil return continuity, while piston compressors are more resistant to short-term oil return fluctuations.
Screw compressors, on the other hand, need to ensure a stable supply of oil return because the oil is involved in sealing and cooling; therefore, when selecting the oil return method, the oil return efficiency corresponding to the type of compressor needs to be accurately matched to avoid wear and tear of the compressor or performance degradation due to the mismatch of the oil return.
Pipe Length and Height ifference
Pipe characteristics directly affect the power and effect of oil return. When the refrigeration system pipeline is long (such as large plant cross-region refrigeration pipeline) or equipment installation height difference is insufficient, the natural driving force of gravity oil return will be greatly reduced, direct oil return is also prone to oil stagnation problems;
And ejector oil return can be used by means of refrigerant high-speed flow of the negative pressure generated by the suction force, break through the limitations of the length of the pipeline and the height difference, and effectively pumping the lubricating oil of the far end or the low position back to the compressor, in this type of scenario has a more In this type of scenario, it is more advantageous.
Operating Conditions
The stability of the operating conditions of the system directly determines the suitability of the oil return method. If the refrigeration system is under the working condition of large load fluctuation (such as supermarket refrigeration system due to changes in customer flow fluctuations in cooling demand, industrial production intermittent refrigeration scenarios, etc.), the oil return load will also change dynamically;
In this case, we need to choose a more adaptable oil return method, to ensure that the oil return can be maintained stably in different working conditions, such as high and low loads, and to avoid interruption of the oil return due to fluctuations in the working conditions or the problem of excessive oil return.
Practical Suggestions
Designers and system integrators are recommended to choose the following options in accordance with the actual needs of the project: gravity oil return for small and simple systems (low-cost and easy maintenance); direct oil return for small and medium-sized systems (streamlining the structure and balancing the cost and efficiency).
Ejector oil return for long pipelines/complex layouts/medium-large systems (to ensure the stability of the efficiency). At the same time combined with the type of refrigerant, oil viscosity and other parameters to optimize the design, maximize the effect of oil return.
Conclusion
Choosing the appropriate oil return method for refrigeration unit is the key to ensure stable operation of the system and reduce energy consumption and maintenance cost. Each of the three oil return methods has its own focus and is suitable for different scenarios.
Combined with the system parameters and working conditions in-depth assessment, the oil return system can effectively support the efficient operation of the unit, avoid the high cost of downtime caused by the oil return problem, and realize the long-term stable operation benefits.
